MC3340 VCA

The Motorola MC3340 VCA chip is used here. It has logarithmic control response. Because it is designed for single supply operation, the output has a DC offset of about half the supply voltage. An op-amp is used to get the control voltage range to 1 – 10 volt. This circuit has very low distortion at moderate signal levels and low signal bleedthrough. But the CV bleedthrough is far too high for synth use. Another problem is that it won't produce 10 V p-p output level. You could add another op-amp at the output to fix this and the DC-offset. But then you would lose the main advantage with this chip, which is low parts count.

Noise & signal attenuation

Red = signal bleedthtrough at 0V CV. Blue = 10V CV, no signal. Green = 0V CV, no signal.

Distortion (THD+N) vs. input level

Frequency response

Test results

Dynamic range 10 V CV, no signal  93 dBr A
  0 V CV, no signal 103 dBr A
  0 V CV, 1kHz 10 V p-p in 78 dBr A
  0 V CV, 2 kHz 10 V p-pin  79 dBr A
  0 V CV, 10 kHz 10 V p-pin  78 dBr A
  Headroom (over 10V p-p)  0 dB
CV bleedthrough no trimming possible 500 mV



In some areas very good performance, but is unacceptable in others. A shame really.


Low distortion at moderate signal levels

Low signal bleedthrough

Low noise

Very high CV bleedthrough

DC offset on output due to single supply operation

Output level too low